1. Field of the Invention
The present invention relates to a focusing method when light beams from a light source is focused on the surface of a photoconductive drum or the like via an optical system, a light beam optical system used for implementing the focusing method and an image forming apparatus using the same. Particularly, the present invention relates to a focusing method using a semiconductor laser or the like for a light source for precisely detecting the dislocation of a focus caused by variation in the light quantity of an optical system due to the change of temperature or aging change and the like, an error in the assembly and adjustment of the optical system and the like, and correcting it with high precision, a light beam optical system used for implementing the focusing method and an image forming apparatus using the same.
2. Description of the Related Art
In a laser beam optical system provided with an optical scanner, light beams outgoing from a semiconductor laser passes through A light beam optical system such as a collimating lens, a polygon mirror and f.theta. lens, is focused and radiated on a photoconductive drum, however, there is a problem that a focal position on the surface of the photoconductive drum is varied by change in the conditions of an optical system due to temperature change or aging change, variation in an irritated position due to the rotational eccentricity of the photoconductive drum, an error in assembly and adjustment and the like, and a predetermined light beams diameter cannot be obtained on the surface of the photoconductive drum. In this case, it is required of correcting light beams diameter by detecting a state in which light beams is focused on the surface of a photoconductive drum or in a predetermined position and shifting a part of an optical system in an optical path, that is, correction for focal dislocation.
For the solution of such a problem, light beams recorder for correcting the focal dislocation of light beams by shifting one optical element constituting an imaging optical system in the direction of an optical axis is disclosed in Japanese Published Unexamined Patent application No. H2-289812.
Referring to FIG. 34, the constitution of the above light beams recorder will be described below. The light beams recorder shown in FIG. 34 is provided with a semiconductor laser 101 for radiating light beams and a collimating lens 102 for collimating diffused light beams radiated from the semiconductor laser 101. The collimating lens 102 is constituted so that it can focus by being moved in the direction of an optical axis by a focusing mechanism 103. Parallel beams outgoing from the collimating lens 102 are reflected and deflected by light beams deflecting system 104. Light beams 105 deflected by the light beams deflecting system 104 is incident on f.theta. lens 106 and focused on the surface of a photoconductive drum 107. The photoconductive drum 107 is provided with an automatic focus (AF) pattern formed range 107a and an image formed range 107b on the surface. A motor 108 rotates the photoconductive drum 107 and outputs a pulse signal in a predetermined cycle according to an encoder.
Light beams reflected in the AF pattern formed range 107a is received by a photodiode 110 via a condenser 109. An AF control circuit 111 operates the focusing mechanism 103 based upon output from the photodiode 110, that is, the diameter of a converged beam on the surface of the photoconductive drum 107, moves the collimating lens 102 in the direction of an optical axis and controls the rotational cycle of the photoconductive drum 107 and the operating timing of the focusing mechanism 103 according to a pulse signal output from the motor 108. A laser driver 112 is used for radiating light beams modulated by the semiconductor laser 101 according to a picture signal and for radiating a fixed quantity of light beams from the semiconductor laser 101 according to an AF operating signal.
The AF pattern formed range 107a is constituted by a reflection type grating pattern and others for reflecting light beams for scanning the surface of the photoconductive drum 107 in the photodiode 110 at predetermined lattice pitch, and the reflection type grating pattern is formed around the photoconductive drum 107 in the vertical scanning direction.
In the above constitution, when a fixed quantity of light beams are radiated from the semiconductor laser 101 according to an AF operating signal, the light beams are deflected by the light beams deflecting system 104 after they are collimated by the collimating lens 102 and scan the surface of the photoconductive drum 107 via the f.theta. lens 106. At this time, light beams reflected in the AF pattern formed range 107a is received by the photodiode 110 via the condenser 109 and a signal according to the beam diameter on the surface of the photoconductive drum 107 is output from the photodiode 110 to the AF control circuit 111. The AF control circuit 111 receives a signal from the photodiode 110 at predetermined timing according to a pulse signal output from the motor 108, moves the collimating lens 102 in the direction of an optical axis by operating the focusing mechanism 103 according to the received signal and corrects the variation of the diameter of a converged beam on the surface of the photoconductive drum 107, that is, the focal dislocation of light beams.
However, according to a conventional type light beams recorder, there is a serious problem that as the quantity of light beams reflected from the photoconductive drum is detected and focal dislocation is corrected based upon the detected quantity, the decrease of reflected light quantity or the unevenness of reflected light quantity in distribution occurs in case toner and others for forming an image adhere to the photoconductive drum and others and hereby, the precision of the correction of focal dislocation is greatly reduced or correction is disabled. The problem that toner and others adhere, as a result, the decrease of reflected light quantity or the unevenness of reflected light quantity in distribution occurs and therefore, the problem that the precision of correction is greatly reduced or correction is disabled is an essential defect which a method of detecting the reflected quantity of light beams reflected from the photoconductive drum has. This is because it cannot be distinguished whether the variation of detected quantity is caused by focal dislocation or by a stain by toner and others.
Also, according to the conventional type light beams recorder, as the quantity of light beams reflected from the photoconductive drum is detected by light beams receiving means arranged in an initialized position, the precision of the correction of focal dislocation is deteriorated in case the position of the light beams receiving means is varied due to temperature and others. This is because as the reflected light quantity of light beams is detected based upon the light beams receiving means arranged in the initialized position, detected quantity also varies when positional relationship between the light beams receiving means and the photoconductive drum is varied. A problem that the precision of the correction of focal dislocation is deteriorated because of the variation of the positional relationship is an essential defect which a focal dislocation detecting mechanism based upon the light beams receiving means arranged in the initialized position has. This is because it cannot be distinguished whether the variation of detected light quantity is caused by focal dislocation or by the variation of positional relationship between the light beams receiving means and the photoconductive drum.
Also, according to the conventional type light beams recorder, if the quantity of dislocation is the same even if the focal position of light beams is dislocated on the side of a light source apart from the photoconductive drum or on the side reverse to the light source, there is a problem that the direction of focal dislocation cannot be distinguished because the diameter of detected converged beams is the same and it takes much time for the focal dislocation of light beams to be corrected.